updated README.md

README.md

+# University of Waterloo Racket Tools
+
+## Graphic Block
+
+Dr.Racket allows non-text elements to be embedded in Racket files.  
+Unfortunately this format is in constant flux, so it's not possible
+to automatically convert this graphical format to plain text so that the code can be run 
+by our testing servers (or read by the TAs doing hand marking).  This is because the servers
+are always much behind the students in terms of which version of Racket they are running.
+
+The graphic-block plug-in for Dr.Racket blocks the user from pasting this content.  This can be 
+toggled in the user's settings.
+
+## HTDP Trace
+
+Racket has a useful tracing tool.  Unfortunately it does not work in Beginning Student.
+This package provides a wrapper that can be used in the HtDP Languages.
+
+To use it, `(require htdp-trace)`
+
+Examples:
+```
+(define/trace (f x)
+ (+ 2 x))
+(f 2)
+>(f 2)
+<4
+4
+
+
+(define/trace (sum-nums n)
+ (cond [(zero? n) 0]
+       [else (+ n (sum-nums (- n 1)))]))
+(sum-nums 3)
+>(sum-nums 3)
+> (sum-nums 2)
+> >(sum-nums 1)
+> > (sum-nums 0)
+< < 0
+< <1
+< 3
+<6
+6
+```
+
+